Liquid composition

ABSTRACT

A liquid composition comprising a solution of a water-soluble metal compound convertible to metal oxide and a smaller amount of a hydrolytically stable water-soluble organic silicone which may be used to prepare binders, coatings and shaped bodies, especially fibres, of intimate mixtures of metal oxide and silica. Preferred silicones are polysiloxane, polyoxyalkylene copolymers and preferred metal compounds are salts of aluminium and zirconium. Phase-stabilised alumina, especially in the form of fibres, is a particularly important embodiment; transitional alumina can be stabilised up to 1400° C without the appearance of separate phases of mullite and crystalline silica.

This is a division, of application Ser. No. 382,198 filed July 24, 1973,now Pat. No. 3,994,740.

This invention relates to a liquid composition and in particular to aliquid composition comprising a metal compound and an organic siliconcompound suitable for the preparation of shaped bodies especiallyfibres, coatings, foams and binders comprising a metal oxide and silica.

Compositions comprising precursors of metal oxide and inorganicprecursors of silica, for example hydrated silica sols, are known, andhave served to produce metal oxide solids, notably alumina and zirconia,containing dispersed silica. It is also known that dispersed silica hasan effect on the phase change properties of alumina. With knownprocesses for incoporating silica into alumina, a significant effect onthe stabilisation of transitional alumina phases is not achieved at hightemperatures. Suppression of the appearance of the alpha form of aluminaat 1400° C can be achieved only by adding sufficient silica so that themajor phase present is crystalline aluminosilicate (mullite). We havenow found that surprisingly metal oxide solids containing dispersedsilica may be produced from compositions comprising precursors of metaloxides and organic silicon compounds. The stabilisation of aluminaphases for example can thereby be effected at temperatures at whichstabilisation was previously not possible. Furthermore stabilisation atlower temperatures can be achieved with lower proportions of silica thanhitherto possible.

According to the present invention there is provided a liquidcomposition comprising an aquous solution of a water-soluble metalcompound decomposable or reactable to produce a metal oxide and of awater-soluble organic silicon compound which is hydrolytically stable inthe liquid composition and in which the silicon atoms are attached tocarbon atoms directly or through an oxygen atom wherein theconcentration of the metal compound expressed as equivalent metal oxideexceeds the concentration of the silicon compound expressed as silicondioxide.

By solution is meant a true solution or a colloidal solution.

Compositions according to the invention are capable of being converted,for example by heating to solids comprising one or more metal oxides andsilica which are suitable for use in the form of foams, binders,coatings, granules, cenospheres, films and especially fibres.

The relative concentrations of metal compound and organic siliconcompound may be varied over wide limits, for example from 1% by weightof silicon compound 99% by weight of metal compound or from 1% by weightof metal compound to 99% by weight of silicon compound. Preferably theweight ratio of the equivalent metal oxide to the equivalent SiO₂ is atleast 85:15.

The metal of the metal compound may be selected from the elements of thePeriodic Table having an atomic number of 4, 12, 13, 20 to 32, 38 to 42,44 to 51, 56 to 60, 62 to 83, 90, 92 or 94. The metals Al, Fe, Zr, Ti,Be, Cr, Mg, Th, U, Y, Ni, V, Mg, Mo, W and Co or mixtures thereof arepreferred: the metals Al, Fe, Zr, Ti and Th and more particularly Al areespecially preferred for fibres made from the compositions.

The anionic constituent of the metal compound may also be selected froma wide range. Two or more compounds of the same or different metals maybe used, if desired. Simple inorganic compounds including thehydroxides; the halides and oxyhalides, especially chlorides andoxychlorides; carbonates; nitrates; phosphates; and sulphates areuseful. Salts of organic acids such as neutral or basic acetates,oxalates, propionates, or formates or organo-metallic compounds are alsosuitable. Basic salts are preferred as they polymerise in solution.Especially preferred are metal compounds which can form a refractoryoxide, especially aluminium oxychloride, basic aluminium acetate, basicaluminium formate, zirconium oxychloride, basic zirconium acetate, basiczirconium nitrate or basic zirconium formate, mixtures thereof or mixedsalts thereof.

The metal compound is most conveniently decomposable to the metal oxideby heating, usually at a temperature from 200° to 1000° C. Carbides ofthe metals may be formed by including carbon or a carbonaceous materialin the composition which, on heating for example, reacts with the metalcompound or a reaction product thereof. In an analogous fashion,nitrides can be formed by including nitrogen-containing compounds in thecomposition. Reaction to form carbides or nitrides may also be broughtabout by the action of carbon- or nitrogen-containing gases on the metaloxide.

The water-solubility of the organic silicon compound is preferablysufficiently high to provide a concentration of at least 0.1% by weightexpressed as SiO₂ dissolved in the liquid composition. The watersolubility may be increased by the inclusion of a water-miscible organicsolvent, for example an alcohol, in the liquid composition.

The silicon compound is preferably a compound containing a monomeric orpolymeric siloxane, silanol or silanolate group, and/or awater-solubilising carbon functional group. By water-solubilising carbonfunctional group is meant a group whch is attached to the compoundthrough carbon and which confers water-solubility on an otherwiserelativey insoluble compound. Examples of such goups are amine, amide,ester alcohol, ether and carboxyl groups. More preferably the siliconcompound is selected from water-soluble polysiloxane-polyoxyalkylenecopolymers. Such copolymers may conveniently be divided into those inwhich the polymer blocks have Si--C linkages and those in which thepolymer blocks have Si--O--C linkages. Si--C linkages are preferred ascopolymers having such linkages are more stable to hydrolysis than thosehaving Si--O--C linkages.

The polysiloxane blocks used in the copolymers preferably contains atleast two siloxane groups of the general type R_(b) SiO_(4-b/2) where bis 1, 2 or 3. Especially useful polysiloxane blocks can contain, forexample, chain terminating groups R₃ SiO₀.5, main chain groups --₀.5OSi(R₂)O₀.5 -- or chain branching groups RSi(O₀.5)₃ or combinations ofsuch groups having the same or different R substituents. It will beunderstood that the O₀.5 units indicate that the oxygen atom is sharedwith a neighbouring Si atom The polysiloxane block may be linear, cyclicor cross-linked, or it can have combinations of these structures. R maybe any monovalent hydrocarbon radical of which examples include alkylradicals such as methyl, ethyl, propyl, butyl, octyl and octadecyl,cycloalkyl radicals such as cyclohexyl, aryl radicals such as phenyl andtolyl an arylalkyl radicals such as benzyl and phenylethyl radicals.Polymethylsiloxane blocks are preferred as they can provide copolymersof highest water-solubiity. A minor proportion of Si-H groups may alsobe present. The polysiloxane block usually has an average molecularweight from 220 to 50,000; molecular weights of preferred blocks are 220to 20000, more preferably 220 to 2000.

The polyoxyalkylene block used in the Si--C linked copolymer may berepresented as

    --R.sup.1 [O(C.sub.m H.sub.2m O).sub.d R.sup.11].sub.χ

in which the --R¹ -- linking group is attached directly to a siliconatom of the polysiloxane by one of its valencies, the other valencies (χin number) being attached to χ number of polyoxyalkylene groups of thetype --O(C_(m) H_(2m) O)_(d) R¹¹. Typical divalent --R¹ -- groupsinclude 1,3-propylene-(CH₂)₃ --, 1,11-undecylene-(CH₂)₁₁ --, ##STR1##Divalent --R¹ -- groups containing C, H and O may also be used, forexample --CH₂ --CH₂ C(O)--, --(CH₂)₃ C(O)--, --(CH₂)₃ OCH₂ C(0)--,--(CH₂)₃ O(CH₂)₂ C(O)--, --(CH₂)₁₁ OCH₂ C(O)--, ##STR2## Typicaldivalent --R¹ -- groups containing C, H, O and N include --(CH₂)₃NHC(O)--, ##STR3## where R = alkyl, cycloalkyl or hydroxyl, ##STR4##Trivalent --R¹ = groups may be used, including ##STR5##

Amongst useful tetravalent --R¹ .tbd. groups are ##STR6## IN theoxyalkylene group (C_(m) H_(2m) O--)_(d) R¹¹, "m" is preferably 2, 3 or4; especially useful oxyalkylene groups are oxyethylene,oxy-1,2-propylene, oxy-1,3-propylene and oxy-1,4-butylene, theoxyethylene especially aiding water-solubility. The oxyalkylene groupsmay be the same or mixtures of oxyalkylene groups may be used. The valueof "d" is preferably chosen so that the molecular weight of the (--C_(m)H_(2m) O)_(d) -- block falls within the range 120 to 9000, especiallywithin the range 400 to 5000.

The terminal group R¹¹ of the polyoxyalkylene chains can be variedwidely. Aliphatic and aromatic groups free from olefinic unsaturationare preferred, for example methyl, ethyl, isopropyl, n-butyl, i-butyl,undecenyl, 2-ethylhexyl, cycloalkyl groups including cyclohexyl, phenyl,tolyl or naphthyl. Other terminal groups R¹¹ which may be used includeacetyl, propionyl, butyryl, carbonate or substituted carbamyl groupssuch as n-penyl-carbamyl C₆ H₅ NHC(O) and n-ethyl carbamyl C₂ H₅ NHC(O).

For copolymers having an Si-O-C linkage between the polymer blocks, R¹in the polyoxyalkylene block is omitted. The polyoxyalkylene groups--O(C_(m) H_(2m) O)_(d) R¹¹ of the Si--O--C linked copolymers are ashereinbefore described.

The oxyethylene content of the polyoxyalkylene is of importance incontrolling the water-solubility of the copolymer. Preferably theaverage C:O ratio in the oxyalkylene chain (--C_(m) H_(2m) O)_(d) isbelow 3:1 to attain sufficient water solubility. We find that about 30%by weight oxyethylene units together with oxypropylene units is usuallyabout the lower limit of oxyethylene which will impart adequate watersolubility to an oxyethylene/oxypropylene polymer, or a copolymer ofthis with a siloxane; the C:O ratio in the oxyalkylene chain in suchcases is about 2.6:1. More preferably, therefore, sufficient oxyethyleneunits should be present in the oxyalkylene block to provide a C:O ratioof at most 2.6:1. The ratio of siloxane to polyoxyalkylene in a methylsiloxane copolymer is preferably less than 2.5:1 for adequate watersolubility.

Examples of water-soluble Si--C linked copolymers useful for thecompositions of the invention are described in United Kingdom PatentSpecifications Nos. 955,916; 1,015,611 and 1,133,273. Examples ofsuitable water-soluble Si-O-C linked copolymers are described in UnitedKingdom Patent Specification No. 954,041.

It is also preferred that the silicon compound be compatible with theother compounds of the liquid composition, especially in notprecipitating a gel or a solid therefrom. Thus, silicon compounds whichare strongly alkaline in water solution are less satisfactory than thosewhich produce a neutral or acidic reaction in water.

The compositions are preferably used at a temperature such that thecloud point of the silicon compound is not exceeded in the liquid.

Other silicon compounds which may be used in the compositions includewater-soluble alkoxy silanes, quaternary and other water-solublenitrogen-containing silanes and siloxanes. Alkali metal siliconates aregenerally sufficiently water-soluble to be useful, for example CH₃Si(OH)(ONa)₂ or CH₃ Si(OH)₂ (ONa). However, such compounds give stronglyalkaline solutions and are less favoured for reasons describedhereinafter.

Since the compositions are usually heated in order to form solidproducts, the silicon compound is decomposable to form silica. It istherefore preferred that the silicon compound should yield the maximumsilica on decomposition, consistent with its other requirements.Suitably, silica yields may vary from 5% to 65% by weight of the siliconcompound. Polysiloxanes solubilised by amine functional groups, forexample --(CH₂)₃ --N(CH₃)₂, can yield above 65%, for example 65% to 75%,by weight of silica.

Additional components such as pigments, polymers, colourants,surfactants, viscosity control additives or source of other oxides, maybe included in the compositions, but since the purpose of the additionalcomponent is related generally to uses of the composition, these aredescribed in more detail hereinafter.

The compositions may conveniently be prepared by dissolving the metalcompound, the silicon compound and any other soluble components in waterin any convenient order. For some embodiments it is necessary to providesome heat to assist dissolution. The compounds may be formed fromsuitable precursors, usually in the presence of the water solvent. Formost uses of the compositions the concentrations of the majorcomponents, for example the metal compound, range from very dilute tosaturation, e.g. in the range 10% to 80% by weight of composition. Theconcentration of dissolved organic silicon compound in the compositionis preferably at least 0.1% by weight expressed as silicon dioxide.

The composition is prepared at any viscosity suitable for the use towhich the composition is put. Viscosities of greater than 0.1 poise, forexample from 0.1 poise to 5000 poise, are generally convenient for useas binders and for the formation of shaped bodies. Viscosity controladditives, for example water-soluble polymers, are useful in producingthe desired viscosity.

It is also possible to use a polymeric metal compound, for example basicaluminium or zirconium salts, to increase the viscosity of thecompositions.

Especially in the case of compositions containing metal compounds whichtend to gel in alkaline conditions it is preferred to maintain a neutralor acid reaction in the composition.

Especially for use of the compositions for making fibres as hereinafterdescribed, a water-soluble silicon-free organic polymer is a muchpreferred additional component of the compositions.

The organic polymer is preferably a non-ionic water-soluble organicpolymer, a polyhdroxylated organic polymer or a natural water-solublegum. The organic polymer is preferably thermally stable under theconditions of fibrising, for example from ambient temperature to withinseveral degrees of the boiling point of water. Examples of preferredorganic polymers include:

partially hydrolysed polyvinyl acetate (polyvinyl alcohol),

polyacrylamide and partially hydrolysed polyacrylamide,

polyacrylic acids,

polyethylene oxides,

carboxyalkyl celluloses, for example carboxymethyl cellulose,

hydroxyalkyl celluloses, for example hydroxymethyl cellulose,

alkyl celluloses, for example methyl cellulose,

hydrolysed starches,

dextrans,

guar gum,

polyvinyl pyrrolidones,

polyethylene glycols,

alginic acids,

polyisobutylene derivatives,

polyurethanes, and

esters, copolymers or mixtures thereof.

Most preferred organic polymers are straight-chain polyhydroxylatedorganic polymers, for example polyvinyl alcohol, partially hydrolysedpolyvinyl acetate, polyethylene oxide or polyethylene glycol.

Conveniently the molecular weight of the organic polymer is in the range10³ to 10⁷, preferably as high a molecular weight as is consistent withthe ability of the organic polymer to dissolve in the solvent used inthe process of the invention. For example, it is preferred for thepolyvinyl alcohol or partially hydrolysed polyvinyl acetate to have amedium or high molecular weight, the polyethylene oxide to have amolecular weight of 10⁴ to 10⁶ and the polymers derived from celluloseto have a molecular weight of 10000 to 50000.

It is preferred that the concentration of organic polymer in acomposition used for forming fibres be from 0.1% to 10% by weight, morepreferably from 0.1% to 2% by weight.

We prefer that little or no chemical reaction should occur between themetal compound and the organic polymer in the fibrising composition.

The especial property of the liquid compositions which makes them usefulfor many technical purposes is their ability to be converted to a solidcomposition in which the metal and silicon compounds remain as anintimate mixture. Many such mixtures are refractory and hard andsuitable for many applications especially those requiring a resistanceto high temperature. Decomposition or reaction of the metal and siliconcompounds to their oxides is usually preceded by at least partial dryingof the composition. Conversion to a solid is conveniently effected byheating, preferably at 200° to 1000° C.

Conversion of the liquid composition may be advantageously carried out,especially for compositions comprising aluminium compounds, bysubjecting the dry or partially dry composition to hydrothermaltreatment, that is, to the simultaneous action of heat and water vapour.Treatment with steam at 250° to 500° C is preferred.

In embodiments where a solid composition is produced from a liquidcomposition comprising a metal compound having an acid anion, forexample aluminium oxychloride, it is especially advantageous to subjectthe solid to the action of a basic substance, for example ammonia or avolatile amine, before, or simultaneously with hydrothermal treatment.

The solid composition may be further heated to change the crystallineform of the oxide phases present or to sinter the composition,preferably at 1000° to 2000° C.

A composition according to the invention may be used to coat substrates,for example glasses, metals, metal oxides or ceramics by applying it tothe substrate and subsequently converting it to form an insolublecoating. The substrate may take a variety of shapes, e.g. fibre,filament, film, granule or powder. Any convenient method, e.g.dip-coating, spraying, roller-or brush-coating, may be used to apply thecoating to the substrate. The coating is dried, at least partly, andpreferably heated, for example to a temperature from 200° to 1000° C todecompose the metal compound and the silicon compound.

The compositions may also be used as a binder or adhesive for a widevariety of materials, especially ceramic or refractory granules andfibres.

The compositions according to the invention are especially useful forthe preparation of shaped bodies, for example cenospheres, films, porousstructures and especially fibres, by forming the composition into thedesired shape and converting the composition to a solid. Shaped bodiesof thin section are preferred, as the release of volatile materials ondecomposition or reaction of the composition is thereby facilitated, andis less likely to lead to cracking failure of the body.

Any convenient method for forming the composition into the desired shapemay be employed; for cenospheres, spray-drying or prilling processes aresuitable; for films, extrusion or casting techniques are convenient; forporous structures, a suitable foaming process or honeycomb formationtechnique may be used; for fibres, any convenient method of fibrisingmay be used, for example centrifugal spinning, drawing, blowing,tack-spining, extrusion through a spinneret or suitable combinationsthereof. A relic process in which the composition is used to impregnatean organic fibre, may also be used. Fibrising by blowing is effected ashereinater described.

The viscosity of the composition used to form fibres is preferably onesuited to the fibrising method employed. Conveniently the viscosity isin the range 0.1 to 3000 poise, preferably 100 to 1000 poise whenfibrising is effected by extrusion of the composition through aspinneret to form a continuous filament. Fibrising of compositions oflow viscosity, for example 0.1 to 100 poise, is preferably carried outby a blowing process as hereinafter described.

It is preferred to remove solvent from the formed body by evaporation,for example by heating at a temperature from 30° to 110° C, optionallyunder reduced pressure.

The shaped body may be further heated to a temperature greater than thatof a drying treatment in order subsequently to complete decomposition ofthe metal or silicon compound, to change the crystalline form of metaloxide phases formed or to sinter the body. Thus the body may be heatedat 1000° to 2000° C, preferably at 1000° to 1500° C, usually for aperiod from one minute to one hour. Heating may be carried out instages, for example in successive steps of increasing temperature.Heating in the presence of air or oxygen may be desirable to oxidise anyorganic material present in the body.

Various additives may be included in the shapd body, singly or incombination, conveniently by adding them to the composition from whichthe shaped body is formed. Additives may also be included on the surfaceof the body by any suitable treatment process. Examples of additiveswhich may be included are:

(a) alkaline earth compounds, for example compounds of magnesium orcalcium, decomposable to alkaline earth oxides;

(b) acid oxides, especially B₂ O₃, P₂ O₅ or ZrO₂ or compounds whichdecompose to form acid oxides;

(c) catalyst materials, for example Pt, Sb, Cu, Al, Psd, Ag, Ru, Bi, Zn,Ni, Co, Cr, Ti, Fe, V or Mn in elemental form or compound form;

(d) fluorides, for example HF, NaF or CaF₂ ;

(e) alkali metal compounds, for example compounds of lithium, sodium orpotassium;

(f) reinforcing particles or fillers such as colloidal silica;

(g) colouring agents, for example mordant dyes or pigments;

(h) rare earth oxides or yttria or precursors thereof.

The catalyst material may be present on the surface of the shaped bodyor it may be included within the body. In some embodiments, the catalystmaterial may be partly within the body and partly on its surface. One ormore catalyst materials may be present.

When at least part of the catalyst material is included in the body, itis convenient to disperse or dissolve the catalyst material, or aprecursor therefor, in the composition from which the shaped body isformed. By precursor is meant a material which when suitably treated,for example by heating or reduction, will generate a catalyst material,directly or indirectly. Shaped bodies, especially fibres comprising acatalyst material may be used in a wide variety of catalytic processesas hereinafter described.

The preferred shaped body is a fibre, conveniently made by fibrising theliquid composition followed by decomposition. Fibrising by extrusionthrough a spinneret is especially useful in producing continuous fibre.Fibrising is most conveniently carried out at the ambient temperature,but if desired it may be carried out at any other temperature at whichthe fibrising composition is stable. For example, it may be convenientin some embodiments to vary the temperature in order to produce theviscosity of the composition appropriate for fibrising.

Fibrising by blowing comprises extruding the fibrising compositionthrough one or more apertures into at least one gas stream having acomponent of high velocity in the direction of travel of the extrudedcomposition. The extruded composition is drawn down by the action of thegas stream on it. The dimensions of the aperture through which thefibrising composition is extruded may vary widely. We prefer to use anaperture having at least one dimension larger then 50 microns andsmaller than 500 microns. The aperture may have a variety of shapes, forexample we have used circular, triangular and star-shaped apertures. Itis convenient in some embodiments to extrude the fibrising compositionthrough a slit, which may be substantially straight or curved, forexample in the case of an annular slit. A plurality of apertures may beused in one extrusion head. The material in which the aperture is formedmay be chosen from a wide variety of substances. A metal, for examplestainless steel or monel, is especially useful. Owing to the fact thatthe fibrising composition may be at or near ambient temperature duringextrusion and that two low extrusion pressures are used, it isconvenient, especially from the point of view of cheapness, to use aplastics material in which to form the aperture; suitable plasticsmaterials includes polystyrene, polypropylene, polyvinyl chloride andpolytetrafluoroethylene.

It is preferred to use two gas streams which converge at or near thepoint where the fibrising composition is extrude from the aperture;preferably the angle between the converging gas streams is from 30° to60°. In preferred embodiments, gas streams emerge from slots on eachside of a row of apertures or a slit; or a conically-shaped gas streamemerges from an annular slot arranged substantially concentricallyaround an annular extrusion slit. The velocity of the gas stream may bevaried over a wide range; we prefer to use velocities in the region of40 to 1500 ft per second. Air is the preferred gas, most convenientlyair at ambient temperature.

Control of the rate of water removal from the extruded composition maybe effected by the degree of saturation of the gas stream. Convenientlythe gas may be mixed with water vapour in the gas reservoir, but asexpansion of the gas from its reservoir may tend to alter the degree ofsaturation, it is sometimes useful to add water-vapour after expansion.Air at a relative humidity of greater than 80% is espcially useful.

The distance separating the point of emergence of the gas stream fromthe extrusion aperture should be as small as possible; we prefer thatthe distance between the closest edges of th aperture and the air slotbe less than 0.25 mm.

The pressure employed to extrude the fibrising composition will dependon the viscosity of the composition, the size and shape of the apertureand the desired rate of extrusion. We find that pressures from 16 to 120lb per square inch absolute are convenient for compositions havingviscosities up to about 100 poise.

The fibre may be dried further after attenuation in the gas stream ifrequired. The fibre may then be subjected to hydrothermal and/or ammoniatreatment as hereinbefore described, if desired. The fibre may alsooptionally be subjected to further processing which may be required, forexample it may be heated to complete the decomposition of the metalcompound to the oxide and to decompose the organic materials in thefibrising composition, to change the crystalline form of oxide phasespresent or to sinter the fibre. Typically, the fibre may be heated at atemperature from 500° to 1200° C for a period of from one minute to onehour, preferably 500° to 800° C for one minute to one hour.

Various additives as hereinbefore described may be included in or on thesurface of the fibre, singly or in any combination, conveniently byadding them to the fibrising composition or by including them on thesurface of the fibre by any suitable treatment process.

Thus the fibres may be coated with a size, such as polyvinyl alcohol orstearic acid. They may be immersed in a solution of ethyl silicate,washed and heated to give a fibre containing extra silica. They may alsobe soaked in solutions of metal compounds, for example magnesiumethoxide in methanol, and the treated fibres heated to give a fibrecontaining additional refractory metal oxide. The fibres may be given asilicone treatment, for example by applying a chlorosilane (in vapour orsolution form) to the fibre surface.

Especially conveniently a catalyst material may be dispersed in thefibrising composition by dissolving it, or its precursor, in the saidcomposition. In preferred embodiments of the invention water-solublematerials, for example salts of catalytic metals, espcially metalnitrates, are dissolved in the aqueous fibrising compositions.

Dispersion of the catalyst material in the fibrising composition mayalso conveniently be carried out by mixing insoluble or partly solubleparticlulate catalyst material with the fibrising composition.Preferably the mean size of particles thus dispersed should be smallerthan the mean diameter of the fibre produced, and more particularlyparticles should be of colloidal size.

Any desired quantity of catalyst material may be dispersed in thefibrising composition provided that the fibre formed is stillsufficiently strong and coherent for use as a fibrous catalyst. We fingthat up to about 10% of a catalyst material may be incorporated in thefibre without serious deterioration in fibre properties.

It is peferred that the catalyst material be chemically compatible withthe constituents of the fibrising composition. When the fibre is heatedas herein dexcribed, it is preferable for the catalyst material to bestable at the temperature of heating. In the case of a catalyst materialprecursor, it is frequently convenient for the catalyst to be formedfrom the said precursor during heating of the fibre.

The catalyst material may be incorporated into the fibre by soaking thesaid fibre in a solution of the catalyst material or a catalyst materialprecursor in a suitable solvent and subsequently removing the saidsolvent from the fibre. Water is a suitable solvent for many catalystmaterials or their precurs, for example metal salts. A fibre may besoaked before or after it is heated to form a fibre of differentcomposition as herein described.

The catalyst material may conveniently be deposited in a suitable formon at least part of the fibre surface. For this purpose it may, ifdesired, be bonded to the said surface by means of a binding agent,which may itself be a catalyst material, for example aluminiumphosphate. Bonding may also be effected by means of an application of acomposition according to the invention of the said surface or to thecatalyst material or both, and removal of the solvent of saidcomposition.

In embodiments in which no binder is used to assist adherence of thecatalyst material to the fibre surface, it is often possible to bringabout some chemical interaction between the catalyst and the fibre toimprove bonding. In most embodiments of the invention, however, it issatisfactory merely to deposit the catalyst material on the fibresurface in a form sufficiently fine that the normal forces of physicalattraction take effect. Thus it is convenient to deposit the catalystmaterial from a mist or vapour comprising the catalyst material or itsprecursor. Most conveniently the catalyst material or its precursor isdeposited on the fibre surface by treating the said surface with adispersion comprising the catalyst material or its precursor and asuitable liquid. A solution of the catalyst material or its precursor ina volatile solvent is espcially useful. In cases where the catalystmaterial is dispersed in a liquid which does not dissolve it, it ispreferred that the catalyst material be in a finely-divided form, mostpreferably having a mean size less than 0.5 micron.

The fibre comprising a catalyst material may be further treated, forexample to bring about desired changes in the catalyst material. Forexample, in cases where a catalyst material precursor has beenincorporated in or on the fibre, it will be necessary to generate theactive catalyst material by a suitable process. The processes normallyused include chemical reaction to form a different compound, reductionand heating. Some of these processes, especially heating, may becombined with hydrothermal treatment or heating the fibre to decomposethe metal compound or the organic material of the fibrising composition.Treatment of the fibre to achieve desirable physical changes in thecatalyst material may also be carried out; for example, changes in thesurface area or crystal structure may be desirable to achieve specificcatalytic effects. Treatment of the fibre to eliminate undesirablesubstances, for example catalyst poisons, may be useful in someembodiments.

The compositions of the invention are espcially useful for thepreparation of coatings, binders and shaped bodies, especially fibres,comprising zirconia or alumina and silica. Thus compositions comprisingan aluminium compound decomposable to alumina, especially aluminiumoxychloride, basic aluminium acetate or basic aluminium formate, and awater-soluble organic silicon compound as herein described may bedecomposed by heating to form solid compositions comprising alumina andsilica in which the alumina is substantially in one or more of itstransitional phase forms at temperatures up to 1400° C.

Alumina is transformed from its transitional phase forms (eta, gamma,delta and theta) to its alpha form on heating at 1200° C for a shorttime ("Alumina as a Ceramic Material" Edited by W. H. Gitzen, TheAmerican Ceramic Society, 1970).

While not wishing to be restricted to any particular theory, we believethat it is likely that the addition of silica to the alumina by the useof a water-soluble silicon compound in the composition of the inventiongives a homogeneous dispersion of the silica in the alumina and therebyproduces a very considerable reduction of the rate of transformation oflow temperature phases to high temperature phases and in particular oftransitional aluminas to alpha-alumina on heating. Thus shaped bodiescomprising alumina prepared according to the invention will exhibitimproved thermal stability.

The invention thus provides a solid composition, for example a fibre,comprising alumina and silica wherein the ratio by weight of alumina tosilica is from 85:15 to 98:2 in which the crystalline alumina issubstantially in one or more of its transitional forms when thecomposition is heated at 1200° C for at least one hour, preferably forat least 10 hours. In such compositions the crystalline alumina istherefore substantially free from the alpha phase.

The invention also provides solid compositions comprising alumina andsilica which, when heated to 1400° C for one minute, two minutes or fiveminutes are substantially free from alpha-alumina and the mullite phaseof aluminosilicate.

The invention further provides solid compositions comprising alumina andsilica which, when heated to 1300° C for 5 minutes, 30 minutes or 2hours, are substantially free from alpha-alumina and the mullite phaseof aluminosilicate.

The invention further provides solid compositions comprising alumina andsilica which, when heated to 1200° C for 10 minutes or one 100 hours,are substantially free from alpha-alumina and the mullite phase ofaluminosilicate.

The invention further provides solid compositions comprising alumina andsilica which, when heated to 1100° C for 1 hour, 10 hours or 100 hours,is substantially free from alpha-alumina and the mullite phase ofaluminosilicate.

Solid compositions comprising alumina and silica when heated to thetemperatures and for the times described are substantially in one ormore of the transitional alumina forms.

Thus solid compositions comprising alumina and silica wherein the ratioby weight of alumina to silicon is from 85:15 to 98:2 may be obtained inwhich the crystalline alumina is substantially in the delta or thetaphase when the composition is heated at 1200° C for at least 1 hour,preferably for 10 hours.

Further, such solid compositions, apart from those in which theta is themajor phase, when heated to the temperatures and for the timesdescribed, show no X-ray crystallographic evidence for the existence ofcrystalline silica or aluminosilicate (mullite).

The introduction of silica into fibres comprising metal oxides by theprocesses of the invention have the further advantage that thehomogeneity of the fibrising composition avoids the problems associatedwith the use of particulate silica, for example as colloidal particles.Such problems include (1) the need to limit fibre diameter due tointerference to required flow characteristics for fibrising by thepresence of particles; (2) the necessity to avoid otherwise desirablepolymeric organic fibrising aids which tend to flocculate sols: and (3)the presence in the finished fibre of regions of high silica contentwhich are liable to crystallise to a silicate phase.

The invention thus provides a fibre comprising silica and alumina and/orother metal oxide, especially zirconia, preferably wherein the ratio byweight of metal oxide and silica is 85:15 to 98:2, preferably from 90:10to 98:3, which may be in continuous or discontinuous lengths, althoughdiscontinuous fibres may have very high ratios of length to diameter,for example greater than 5000. Fibres can be made with average diametersless than 20 microns, typically from 0.5 to 5.0 microns. As a result ofthe avoidance of formation of undesirable crystal forms of alumina ashereinbefore referred to, alumina fibres show remarkable resistance tophysical change at high temperature, for example from 1000° to 1400° C.In general, the fibres heated at 500° to 800° C have a very high surfacearea, a BET surface area of more than 50 m² /g being consistentlyobserved, and figures of 50 m² /g to 200 m² /g being the usual measuredrange after hydrothermal treatment and after 1 hour of heating at 500°to 800° C. The presence of silica introduced by the processes of theinvention increases the thermal and hydrothermal stability with respectto surface area of an alumina fibre, and hence certain catalyticproperties. The acidity of the alumina conferred by the silica contentprovides improved ion exchange properties of the solid composition. Thefibres may be collected as individual fibres or they may be collected inthe form of a yarn, mat or felt. Mats or felts are conveniently formedby collecting the fibres on a moving band, preferably a band offoraminous material, for example steel mesh. The fibres may be collectedon a mould to provide a shaped felt. If desired the fibres may be bondedtogether, for example by collecting the fibres before they are dry andheating the resultant mat or felt. Bonding may also be effected by theuse of a binding agent. The mat or felt may be compressed, if desired,for example to increase its density. The invention is especially usefulin preparing glassy fibres. Fibres spun into yarn may be made up ascloth.

Fibrous catalysts according to the invention comprising the metalscopper, ruthenium, nickel, palladium, platinum or silver, compounds orcombinations thereof, are especially useful in processes such as thefollowing:

dehydration of alcohols,

methanol synthesis,

reduction of nitrobenzene,

ammonia decomposition,

steam reforming of naphtha or natural gas,

hydrogenation of olefins, aromatics, nitrides, fats and oils,

sulphur dioxide oxidation,

hydroalkylation,

methane ammoxidation,

ethylene oxide from ethylene,

formaldehyde from methanol.

Semiconductor oxides are useful catalyst materials. For example, Cr₂ O₃/"eta" alumina may be used for paraffin dehydrogenation or naphthareforming.

Metallic halides, for example CuCl₂, SbCl₃, AlCl₃ or CrCl₃, providefibrous catalysts which are useful for a variety of chlorination andoxychlorination reactions or isomerisation of paraffins, olefins andaromatics.

Organo-metalic catalysts may be best employed in the invention bysoaking or coating of the preformed fibre. The fibrous catalysts areuseful in producing ethylene oligomers, polyethylenes and polyesters.Metal carbonyls, for example HCo(CO)₄, provide fibrous catalystssuitable for carrying out OXO processes.

The fibrous catalysts, especially those containing platinum, palladium,molybdenum, Co₃ O₄, V₂ O₅, Cr₂ O₃, MnO₂, Fe₂ O₃ or NiO, or combinationsthereof, may be used in a car exhaust treatment device, for example tocatalyse the oxidation of car exhaust gases, for example in anafterburner.

Other catalytic materials found useful include:

cobalt molybdate,

nickel molybdate,

bismuth molybdate,

copper molybdate,

zinc chromite,

cobalt oxide, Co₃ O₄.

Fibrous catalysts according to the invention are advantageous owing totheir high external surface areas; they are heat-resistant andmechanically strong.

The invention is thus useful in producing shaped bodies comprising metaloxides, especially fibres and more especially alumina fibres which maybe of very small diameter, dense, white, strong and of high modulus forexample 20 × 10⁶ to 35 × 10⁶ pounds per square inch Young's modulus inthe case of alumina fibres. The bodies, especially the fibres, mayconventiently be used, for example as high temperature insulatingmaterials, fillers, as reinforcement for resins, metals and ceramicmaterials, inert filters, catalysts or catalyst supports.

The invention is illustrated, but not limited, by the followingExamples. All X-ray diffraction results quoted in the Examples wereobtained using a Philips diffractometer, incident Copper radiation and agrahite monochromator in the diffracted beam to select the K αwavelength. Apparent crystallite sizes of eta alumina were derived fromthe measured full width at half height of the diffraction maxima at 67°2 θ after instrumental broadening had been removed by fourierdeconvolution, (an apparent crystallite size of 60 A is derived from apeak at 67° 2 θ having a deconvoluted full width at half height of 1.76°2 θ). Phase identification is based on the results of J. W. Newsome, H.W. Heiser, A. S. Russell, H. C. Stumf, Technical Paper No. 10, secondrevision, Alumina Properties, Aluminium Company of America, Pittsburg,Pennsylvania, 1960.

In the following Examples reference is made to various siliconcompounds. Details of these are shown in the following tables, with thecorresponding reference used in the Examples.

    __________________________________________________________________________                                          Silicone to                                                              Approx                                                                             polyether ratio                         Ref.                                                                             Structure                     MW   w/w                                     __________________________________________________________________________        ##STR7##                     12,000                                                                             1:2.3                                   B                                                                                 ##STR8##                     2,400                                                                              1:2.3                                   C                                                                                 ##STR9##                     2,500                                                                              1:2.3                                    D*                                                                               ##STR10##                    12,000                                                                             1:2.3                                   E                                                                                 ##STR11##                    4,700                                                                              1:0.7                                   F                                                                                 ##STR12##                    900  --                                      __________________________________________________________________________     *diluted with 100:7 of C.sub.4 H.sub.9 (OC.sub.2 H.sub.4).sub.17 (OC.sub.     H.sub.6).sub.13 . OH                                                     

                  Table 2                                                         ______________________________________                                                                          Silicone to                                 Refer-                    Approx  Polyether                                   ence   Spurce             MW      ratio                                       ______________________________________                                        DC192  Dow-Corning Corporation                                                                          25,000  1:2.6                                       DC193  Dow-Corning Corporation                                                                           2,100  1:1.7                                       L546   Union Carbide Corporation                                                                        15,000  1:2.6                                       L5340  Union Carbide Corporation                                                                        14,000  1:1.5                                       ______________________________________                                    

EXAMPLE 1

Fibres were prepared from a solution of the following composition.

200g Aluminium chlorohydrate. (Al:Cl ratio 2:1 23.8% w/w Al₂ O₃)

94.4g Polyvinyl pyrrolidone solution (3% w/w of k-90 grade to give 6%w/w Al₂ O₃)

9.3g Polysiloxane copolymer A (Table I)

The solution was evaporated under partial vacuum at 35°-40° C until theviscosity at ambient temperature was 15 poise. The solution was extrudedthrough small holes 240 microns diameter) and attenuated with air togive dry unfired fibres with a mean diameter of 4 microns.

The fibres were further dried at 100° C, heated in steam at 350° C for15 minutes and fired at 900° C for 15 minutes to give a strong whiteflexible product.

Chemical analysis showed that the fibre contained 5% SiO₂ (w/w Al₂ O₃).Electron microscope examination showed no evidence of oxide phases otherthan the transition alumina phases.

Samples of fibres were heated to the temperature given below. Surfacearea measurement by the nitrogen BET method gave the following results:

    ______________________________________                                        Temp. ° C                                                                              Time (hr)     SA m.sup.2 /g                                   ______________________________________                                        1100            1             61                                              1200            1/2           40                                              ______________________________________                                    

X-ray phase analysis showed that fibres heated for 65 hours at 1200° Cwere in the gamma and delta alumina phases, with only a trace of thealpha phase detected. Fibres heated for 1 hour at 1350° C were in amixture of the gamma, delta and theta phases, with no trace of alphaalumina.

EXAMPLE 2

Fibres were prepared as in Example 1, but with only 4.6g of thepolysiloxane copolymer A to give 2% SiO₂ w/w Al₂ O₃. Fibres fired asdescribed in Example 1 up to 900° C and then fired for 1 hour at 1200° Cmaintained flexibility and strength. X-ray analysis showed only thegamma and delta phases.

EXAMPLE 3

Fibres prepared as in Example 1, but containing 5% SiO₂ w/w Al₂ O₃ fromthe polysiloxane copolymer B instead of A were fired as in Example 1 to900° C. Nitrogen absorption measurements gave a surface area of 74 m² /gand a pore volume of 0.083 cm³ /g. The only phase present on heating to1000° C for 2 hours was eta alumina. After heating to 1100° C for 16hours the major phase present was gamma alumina with a trace of deltaalumina.

No evidence was obtained for an aluminosilicate or silica phase.

EXAMPLE 4

Solutions were prepared as in Example 1 using the commercially availablewater-soluble polysiloxane copolymers DC 192, DC 193, L 546 and L 5340listed in Table II. Appropriate quantities were added to give 2% silicain the final alumina fibres which were spun and heat-treated as inExample 1 up to 900° C. After calcination at 900° C the fibres werestrong and flexible, with no evidence of a mullite or alpha aluminaphase by X-ray investigation. X-ray analysis on fibres heated to 1050° Cfor 1 hour gave only transition alumina phases.

EXAMPLE 5

Fibres were prepared as in Example 1 using the polysiloxane copolymer Cto give the following combinations of compositions, - all percentageweights expressed on the Al₂ O₃ content of the fibres or solutions:

5.1; 5% SiO₂ 6% Polyvinyl pyrrolidone

5.2; 5% SiO₂ 6% Polyvinyl alcohol -- high molecular weight water-solublegrade

5.3; 1% SiO₂ 6% Polyvinyl pyrrolidone (PVP)

5.4; 7.5% siO₂ 6% PVP

after heat treatment as in Example 1 to 900° C all fibres were strongand flexible.

EXAMPLE 6

A sample of the fibres prepared in Example 3 and heated to 900° C weresandwiched between `Incoloy` alloy DS metal gauzes and tested for 50hours in a car exhaust stream on an engine test bed at temperatures upto 750° C. At the end of the test the total time at 750° C was 20 hours,with the majority of the remainder at 500° C. Inspection of the fibrouspad after this treatment showed that the fibre had not sufferedsignificant damage or weight loss. X-ray analysis after treatment showedthe only phase present was eta alumina with a crystallite size less than80 A. The surface area after treatment had increased to 110 m² /g.

EXAMPLE 7

Fibres were prepared as in Example 1 but with a 5% nickel content, addedto the solution as NiCl₂ .6H₂ O. On firing as in Example 1 to 900° Cstrong fibres were produced.

EXAMPLE 8

Fibres were prepared as in Example 1 with 5% MgO content, added asMgCl₂.6H₂ O to the spinning solution. On firing to 900° C as in Example1 strong white fibres were produced. When heated in a hot-stage X-rayapparatus, the fibres showed the presence of alpha alumina at 1140° C.

EXAMPLE 9

Fibres were prepared to give a 1:1 w/w ratio of ZrO₂ :Al₂ O₃ with 7% w/wbased on ZrO₂ of rare earth oxides (60% yttria grade) as a zirconiaphase stabiliser and containing 5% w/w of slica. The spinning solutionwas prepared from the following components:

Aluminium oxychloride solution (23.5% Al₂ O₃ w/w, Al:Cl, 2:1)

Zirconyl acetate solution. (Commercial 22% w/w ZrO₂ grade)

Rare earth chlorides solution (60% yttria grade)

Polyethylene oxide (3000,000 MW)

Polysiloxane copolymer B

The solution was evaporated, spun into fibres and fired as in Example 1.After 1 hour at 1200° C the fibres were strong and flexible. Incomparison fibres prepared from a similar solution without the siloxaneadditive were weak and friable when fired to 1200° C.

EXAMPLE 10

Fibres were prepared as in Example 1 but using the siloxane copolymer Dto give 5% SiO₂. The solution prior to spinning was cloudy and theunfired fibres produced by extrusion followed by attenuation with an airstream were of variable geometric quality. The fibres were heated insteam at 350° C. for 15 minutes, followed by calcination at 1200° C for1 hour to give white flexible fibres. X-ray analysis showed the fibrescontained eta, gamma and delta alumina with a minor amount of alphaalumina. The presence of alpha alumina is thought to be due to phaseseparation (cloudiness) resulting in a lower effective SiO₂ content insome parts of the fired fibre.

EXAMPLE 11

A spinning solution was prepared from the following components:

200g Aluminium chlorohydrate (23.8% Al₁ O₃ by weight Al:Cl. 2:1)

2.85g Polyvinyl alcohol (high molecular weight water-soluble grade)

140g Water

9.4g Polysiloxane copolymer C

The solution was evaporated to a viscosity of 15 poise measured atambient temperature and spun into fibres with a mean diameter of fourmicrons. The fibres were dried at 100° C, heated in steam at 350° C for15 minutes and calcined at 900° C for 15 minutes. A sample of thesefibres was heated to 1200° C on a hot-stage X-ray diffractometer at arate of 30° C/hour. Phases present at 800° C were chi and eta alumina;the gamma phase appeared at 1020° C and reached a maximum at 1110° C;the delta phase appeared at 1060° C and reached a maximum at 1140° C,while the eta phase had faded by 1150° C. There was no evidence formullite or alpha alumina at 1200° C, or on re-cooling to roomtemperature.

EXAMPLE 12

A solution suitable for use as a binder or coating material was preparedby co-dissolving the following components;

400g Aluminium chlorohydrate solution (2:1 Al:Cl ratio, 23.5% w/w Al₂O₃)

1g Glacial acetic acid

10g Polyvinyl pyrrolidone (K 60 grade)

24g Siloxane copolymer A (23.5% w/w SiO₂)

200g of a graded tabular alumina grog were well-mixed with 20g of theabove solution and pressed into a glass dish. The contents of the dishwere subjected to ultrasonic vibration for 30 minutes, and allowed toset for 20 hours in a drying oven at 80° C. The tablet so formed wasremoved from the dish and heated to 400° C over a 2 hour period, andsubsequently to 900° C over a further 2 hours. The tablet was thentransferred directly into a furnace at 1400° C and heated at thattemperature for 1 hour. After the tablet was removed from the furnaceand allowed to cool to room temperature, it was examined and found to betough and free from surface cracks.

EXAMPLE 13

A 1-inch cube of ceramic honeycomb, suitable for use as a car-exhaustceramic matrix component, was coated with high-surface area aluminausing the solution prepared as described in Example 12. The untreatedcube had a specific BET surface area of 0.4 m² /g, an estimated flatgeometric area of 200 cm², and a weight of 13g. This cube was soaked inthe solution described above while suspended on a fine wire, and thenslowly withdrawn from the solution over a period of 5 minutes. The cubewas placed on a filter paper pad to allow residual solution to drainoff, and was dried at 80° C for 10 minutes. The cube was subsequentlyheated in air at 350° C for 10 minutes and fired at 900° C for 15minutes. On cooling the weight increase was found to be 0.6g and the BETsurface of the cube was 4.8 m² /g.

EXAMPLE 14

To 80g of the solution prepared as described in Example 12 was added 1gof chloroplatinic salt (0.4g Pt). A ceramic honeycomb cube as used inExample 13 was coated in this solution, drained, dried and fired to 900°C as described in Example 13. The increase in weight of the cube wasfound to be 0.8g, and the BET surface of the cube was now found to be4.3 m² /g.

EXAMPLE 15

The silicone-polyoxyethylene compound E having a calculated silicaequivalent of 46.5% was found to give a cloudy mixture when added to analuminium oxychloride solution at a viscosity of 10 poise, which settledout to give two distinct layers of solution. Accordingly compound E wasmixed at a 50:50 volume ratio with industrial methylated spirits, andthis solution was carefully mixed with a 20 poise solution containing 1%w/w polyethylene oxide (molecular weight 300,000) and 28% w/w Al₂ O₃ asbasic aluminium oxychloride, to give a clear bubble-free mixture. Thismixture was extruded through small holes into co-current streams ofhigh-velocity humidified convergent air jets and collected on a wireguaze as fibre having a mean diameter of 6 microns.

EXAMPLE 16

An amine-functional polysiloxane copolymer F contained a `silicaequivalent` of 66.3%.

1.79g of this compound were mixed with an equal volume of industrialmethylated spirits and then titrated with N/10 hydrochloric acid to givea pH reading of 4.0. This solution was then mixed with 36.7gpolyethylene oxide solution (2% w/w of molecular weight 300,000) and100g of 23.8% w/w Al₂ O₃ equivalent aluminium oxychloride solution. Thefinal mixture was evaporated down to a viscosity of 20 poise in a vacuumevaporator and blow-spun into fibres having diameters in the range 2-3microns.

EXAMPLE 17

4.76g of t-butoxy silatrane were dissolved in water and filtered toremove a trace of insoluble material. The pH of the solution wasadjusted to 4.0 with dilute hydrochloric acid and the solution was mixedwith 100g of 5/6 basic aluminium oxychloride solution (23.5g Al₂ O₃) and36.7g of a 2% aqueous solution of 300,000 molecular weight polyethyleneoxide. The mixture was evaporated to a viscosity of 30 poise on a vacuumrotary evaporator to give a clear yellowish solution. A sample wasallowed to thicken by evaporation in a glass dish and fibres were pulledfrom the sample at the end of a spatula. On standing for 20 hours asample was found to have set to a gelatinous material. The fibresobtained were treated with ammonia, heated in steam at 350° C and firedat 900° C for 15 minutes followed by 1200° C for 1 hour.

EXAMPLE 18

Alumina fibres containing silica and boric acid were prepared from thefollowing composition:

200g Aluminium chlorohydrate (Albright & Wilson, 23.5% Al₂ O₃, 2:1 Al:Clratio)

2g Boric acid

5.6g Polysiloxane copolymer A

143g Polyethylene oxide (1% w/w 300,000 molecular weight)

The solution was evaporated to a viscosity of 20 poise and blow-spuninto fibres having a mean diameter of 4 microns. The fibres were treatedwith 0.2% v/v ammonia in air at ambient temperature, heated in steam at350° C for 1/4 hour and fired at 900° C for 1/4 hour, and finally at1300° C for 1 hour. Phase analysis by X-ray diffraction showed that themajor phase was delta alumina with a minor theta alumina phase and asmaller amount of an unidentified phase. No trace of alpha alumina ormullite was detected. A further sample of the fibre fired to 900° C wasreheated at 1400° C for 1 hour and found to contain a major thetaaluminium phase and a minor alpha alumina phase: again mullite was notdetected.

EXAMPLE 19

Fibres were prepared from the following formulations:

200g aluminium oxychloride (23.5% w/w Al₂ O₃. Albright & Wilson)

138g Polyvinyl alcohol solution, 2% w/w `Elvanol' 50-42 in water

9.2g Polysiloxane copolymer A

The solution was evaporated to a viscosity of 100 poise and allowed tostand for 20 hours. The solution was then extruded from a bomb through amicrofilter and out of a 100 micron spinerette hole. Fibres were drawndown and wound up on a rotating drum covered in polythene film. Thefibres were removed from the drum, dried at 100° C, heated in steamcontaining 5% v/v ammonia at 350° C for 1/2 hour and fired at 1000° Cfor 1/2 hour. Samples of this fibre were fired at 1200°, 1300° and 1400°C for 1/2-hour periods. Selected fibres, with diameters approximately 10microns were mounted on the head of a radio loud speaker using sealingwax. The modulus of the fibres was measured using the vibrating readtechnique for lengths in the range 0.2 to 0.4 cm. The loud speaker wasfed from a decade oscillator and the resonant frequency of the fibreswas measured by observing the vibration at resonance frequency using atravelling microscope. A graph was plotted for each fibre of thefundamental resonance frequency fr against D/l², where D = diameter andl = length are found to be a straight line passing through the originwithin expected error. The specific modulus of the fibre (E/ρ) was thencalculated from the slope M using the formula E/ρ = 0.074 M² psi, whereρ = fibre density.

The results for the relative specific modulus are given below, comparedwith that of E-glass fibre (E/ρ = 1)

    ______________________________________                                        Firing Temperature (1/2 hr) ° C                                                            1000   1200    1300 1400                                  Relative Specific Modulus (E/ρ)                                                               1      1.3     1.5  2.3                                   ______________________________________                                    

For comparison, similar fibres made from a formulation not containingthe copolymer and fired at 900° C gave an E/ρ value of 1.5, but onfiring at 1000° C this dropped to 1 and at high temperatures the fibreswere too brittle for measurements to be obtained.

EXAMPLE 20

Fibres were prepared from the following components:

200g Aluminium chlorohydrate (23.8% w/w Al₂ O₃, 2:1 Al:Cl ratio)

142.8g Polyethylene oxide solution (1% w/w 300,000 MW)

18.8g Polysiloxane copolymer A

The solution was evaporated to a viscosity of 15 poise and blow-spuninto fibres having a mean diameter of 3 microns. The fibres werecollected and treated with 0.2% v/v ammonia gas in air, heated in steamat 350° C for 1/4 hour and fired at 900° C for 1/4 hour. Chemicalanalysis indicated that the fibres contained a ratio of SiO₂ :Al₂ O₃ of9:100.

A sample of fibre was heated for 1 hour at 1200° C. X-ray analysisindicated that the fibres contained a major delta alumina phase and aminor gamma phase. No alpha alumina or mullite was detected.

A further sample was fired at 1300° C for 1 hour. This sample gave majorphases of mullite and theta alumina. On firing a further sample to 1400°C for 1 hour major phases of mullite and theta alumina were present,with a trace of alpha alumina detected.

EXAMPLE 21

Fibres were prepared as in Example 20, but with only 4.7g of thecopolymer A. These fibres on heating for 1 hour at 1200° C gave majorgamma and delta phases with eta alumina also present. On firing for 1hour at 1300° C major alpha and theta phases of alumina were observed.After 1 hour at 1400° C a major alpha alumina phase and the mullitephase of alumino silicate was observed.

In comparison, a similar fibre of eta alumina containing no silica gavea major alpha alumina phase after 1 hour at 1200° C. However a majordelta phase could be obtained by heating the pure alumina fibre when inits eta phase in an atmosphere free from chloride for 2 hours at 1050°C. Heating the pure alumina fibre at 1200° C for a further 1/2 hour gavealpha alumina.

EXAMPLE 22

Fibres were produced as in Example 20, but with the requisite amount ofcopolymer A to give 5% silica w/w total oxides in the final fired fibre.The fibre was fired for periods of 1-20 hours at 1000° C. Phase analysisshowed that after 20 hours the phase was eta alumina with an apparentcrystallite size of 60 A. A sample of fibre was set in epoxy resin andion-beam thinned with argon ions to give a specimen suitable fortransmission election microscope (TEM) studies. Examination of themicrostructure by TEM with modifications of up to 200,000 showed an evengranular structure. No evidence was obtained for the presence of morethan one crystalline phase, nor was there any evidence for anon-homogeneous dispersion of silica or silicate particles.

A sample of fibres fired for 20 hours at 1200° C gave a major deltaalumina phase by X-ray analysis. An ion-beam thinned sample showed agranular microstructure similar in form to that previously described,although dark-field images showed larger crystalline regions of theorder of 500 A in size.

Further X-ray studies on the samples indicated the degree ofcrystallinity of the delta alumina was greater than 25% by weight, andthe intensity of background scattering was consistent with an amorphouscomponent in the order of 25% by weight.

EXAMPLE 23

Fibres were produced as in Example 20, but with sufficient silicacontent from the copolymer A to give 7% by weight of SiO₂ :Al₂ O₃. Afterthe 900° C firing stage, these fibres were refired for 3 minutes at1500° C in a tube furnace. Transmission electra microscopy on an ionbeam thinned sample at a magnification of × 100,000 single crystalplatelets apparently set in a glassy matrix. Examination of a sample ofthis fibre of X-ray analysis showed that the fibre contained a majortheta alumina phase and a minor mullite phase. Stereoscan micrographs(Cambridge Stereoscan S2A) of these fibres at a magnification of 10,000showed an apparently smooth fibre surface.

A further sample of these fibres heated to 1600° C for 5 minutes hadconverted to a major alpha alumina phase with a minor mullite phase.Transmission election microscopy showed anisometric single crystals ofalpha alumina with one dimension of ˜2000 A and a second dimension of upto 2 microns.

A furthe sample in the form of a fibre blanket was heated at 1400° C for20 hours. These fibres had developed a surface roughness when observedwith the Stereoscan at a magnification of 5,000. Fibres of pure aluminafrom a similar formulation but without the silia, when heated in anidentical manner gave blankets of lower resilience, and on Stereoscanexamination showed surface features corresponding to the formation oflarge alpha alumina crystals which in many places crossed the visiblesurface.

EXAMPLE 24

Fibres produced as in Example 23 were heated in air for 1 hour at 1200°C and shown to contain a major delta phase of alumina with no trace ofmullite or silica phases.

In comparison, a sample of commercial alumina/silica catalyst wasanalysed and found to have a similar composition (3.8% Si, 42.2% Al).Harshaw alumina AL-1605P-L2621-35-31 was found to give a small amount ofcristobalite and a trace of mullite when fired for 1 hour at 1200° C.Furthermore, after 5 minutes at 1400° C the sample contained a majoralpha alumina phase, together with theta alumina and mullite.

EXAMPLE 25 (Comparative Example)

Fibres were prepared from the following components:

200g Basic aluminium oxychloride solution (Al:Cl ratio 1.7:1, 23% Al₂O₃)

8.8g Silica sol (`Ludox` AM) (28.5% SiO₂)

The solution was evaporated down to a viscosity of 80 poise on a rotaryvacuum evaporator, and spun on a centrifugal spinner at 2500 rpm throughsmall peripheral holes to give fibres with a mean diameter of 10microns. The fibres were dried at 80° C for 6 hours, heated in steam for8 hours at 350° C and fired to 1000° C for 1/4 hour. X-ray analysisshowed that these fibres contained the eta and chi phases of alumina,but on heating in a hot-stage X-ray diffractiometer up to 1200° C, inaddition to transition alumina phases, mullite was observed from 1160°C. Fibres heated to 1200° C were weak in comparison to those produced asin Example 22 and heated in a similar manner.

What we claim is:
 1. A process for the preparation of a fiber comprisinga metal oxide and silica comprising the steps of(a) providing a liquidcomposition having a viscosity of greater than 0.1 posise comprising anaqueous solution of a water-soluble metal compound and a water-solubleorganic silicon compound which is stable to hydrolysis in the liquidcomposition and in which silicon atoms are attached to carbon atomsdirectly or through an oxygen atom and wherein the concentration of themetal compound expressed as equivalent metal oxide exceeds theconcentration of the silicon compound expressed as silicon dioxide; (b)fiberizing the said liquid composition to form fibers; and (c) heatingthe said fibers to decompose the metal compound and the silicon compoundto oxides.
 2. A process as claimed in claim 1 wherein the weight ratioof metal compound expressed as equivalent metal oxide to siliconcompound expressed as silicon dioxide is at leat 85:15.
 3. A process asclaimed in claim 1 wherein the metal of the metal compound is selectedfrom the group consisting of aluminum, iron, zirconium, titanium,beryllium, chromium, magnesium, thorium, uranium, yttrium, nickel,vanadium, molybdenum, tungsten and cobalt.
 4. A process as claimed inclaim 1 wherein the metal compound is selected from the group consistingof metal hydroxide, halide, oxyhalide, carbonate, nitrate, phosphate,sulphate and salts of an organic acid.
 5. A process as claimed in claim4 wherein the salt of an organic acid is selected from the groupconsisting of a basic formate, acetate, oxalate and propionate.
 6. Aprocess as claimed in claim 1 wherein the metal compound is selectedfrom the group consisting of aluminum oxychloride, basic aluminumacetate, basic alumium formate, zirconium oxychloride, basic zirconiumacetate, basic zirconium nitrate and basic zirconium formate.
 7. Aprocess as claimed in claim 1 wherein the silicon compound contains asilanol or silanolate group or a water-solubilizing carbon functionalgroup or mixtures of these groups.
 8. A process as claimed in claim 1wherein the silicon compound contains a siloxane group and awater-solubilizing carbon functional group.
 9. A process as claimed inclaim 8 wherein the silicon compound is a water-solublepolysiloxane-polyoxyalkylene compolymer.
 10. A process as claimed inclaim 9 wherein the polysiloxane and polyoxyalkylene blocks of thepolysiloxane-polyoxyalkylene copolymer are linked by Si--C linkages. 11.A process as claimed in claim 9 wherein the polysiloxane block of thpolysiloxane-polyoxyalkylene copolymer is a polymethylsiloxane block.12. A process as claimed in claim 9 wherein the molecular weight of thepolysiloxane block of the polysiloxane-polyoxyalkylene copolymer is from220 to 20,000.
 13. A process as claimed in claim 9 wherein thepolyoxyalkylene block of the polysiloxane-polyoxyalkylene copolymercomprises oxyethylene groups.
 14. A process as claimed in claim 13wherein the ratio of carbon atoms to oxygen atoms in the oxyalkylenechain is below 3:1.
 15. A process as claimed in claim 9 wherein thepolyoxyalkylene block comprises additionally oxypropylene groups and theoxyethylene groups comprise at least 305 by weight of thepolyoxyalkylene block.
 16. A process as claimed in claim 11 wherein theratio of polysiloxane to polyoxyalkylene is less than 2.5:1.
 17. Aprocess as claimed in claim 1 wherein the liquid compositionadditionally comprises a water-soluble silicon-free organic polymer. 18.A process as claimed in claim 17 wherein the organic polymer is selectedfrom the group consisting of partially-hydrolyzed polyvinyl acetate,polyvinyl alcohol, polyvinyl pyrrolidone or polyethylene oxide.
 19. Aprocess as claimed in claim 1 wherein the fiber is dried prior todecomposition of the metal and silicon compounds to their oxides.
 20. Aprocess as claimed in claim 1 wherein the fibers are heated at atemperature of 500° to 1200° C for one minute to one hour.
 21. A fibercomprising a metal oxide and silica whenever prepard by a process asclaimed in claim
 1. 22. A fiber as claimed in claim 21 wherein the ratioby weight of metal oxide to silica is at least 85:15.
 23. A process asclaimed in claim 1 wherein fiberizing is effected by extrusion of theliquid composition at a viscosity of 100 to 1000 poise through aspinneret to form a continuous filament.
 24. A process as claimed inclaim 23 wherein fiberizing is effected by blowing.
 25. A process asclaimed in claim 24 wherein the liquid composition has a viscosity of0.1 to 100 poise.
 26. A process as claimed in claim 25 wherein blowingcomprises extruding the liquid composition through one or more aperturesinto at least one gas stream having a component of high velocity in thedirection of travel of the extruded composition whereby the extrudedcomposition is drawn down.
 27. A process as claimed in claim 26 whereinthe liquid composition is extruded into two gas streams which convergeat or near the point when the composition is extruded from the aperture.28. A process as claimed in claim 27 wherein the gas is air.
 29. Aprocess as claimed in claim 28 wherein the air is at a relative humidityof greater than 80%.
 30. A process as claimed in claim 1 wherein thefibers are heated with steam at 250° to 500° C.
 31. A process as claimedin claim 30 wherein the fibers are subjected to the action of ammonia ora volatile amine before or with the heating with steam.
 32. A process asclaimed in claim 31 wherein the fibers are further heated at atemperature of 1000° to 2000° C to change the crystalline form of theoxide phases present in said fibers or to sinter the same.